ATCRBS and IFF radar beacon system interrogators are troubled by the occurrence of many false reply signals that are indistinguishable from valid reply signals on the basis of proper spacing of framing pulses and similar criteria. Such false signals may be the result of transponder responses to interrogations by a second radar, or may result from overlapped responses by two or more transponders to the same interrogating radar. In an IFF system, false signals may be caused intentionally by a hostile party attempting to mislead the interrogating party.
False reply signals that are passed by bracket decoders, i.e., those reply signals having properly spaced framing pulses, are known in the art as fruit and the means for discriminating against such signals are known as defruiters. Defruiters operate on the principle that fruit occurs asynchronously so that replies which do not appear successively at consistent ranges can be rejected as false replies. In an IFF system, replies may be delayed by an amount specified in an encoded interrogation signal in order to conceal the true range of the responding targets from unfriendly forces. Encoded interrogations provide a means for validating the authenticity of replies because only friendly forces can decode the requested delay value and respond with a correctly delayed reply. Such security delays, however, complicate the defruiting process.
U.S. Pat. Nos. 3,182,310, issued May 4, 1965 and 3,302,197, issued Jan. 31, 1967, both to B. H. Humpherys, disclose sequential defruiters in which bracket decoded reply signals to a first interrogation signal are input to a delay line having a length equal to the pulse repetition interval (PRI), i.e., the interval between interrogation signals. Bracket decoded reply signals to the interrogation signal next following the first are compared as they are received with overflow outputs from the delay line. Those bracket decoded reply signals received during the second PRI that coincide with outputs from the delay line are accepted as valid reply signals.
The defruiters of the '310 and '197 patents have the disadvantage of requiring bracket decoded reply signals to be time coincident for two successive interrogation signals to be declared valid, although it is possible for valid reply signals to be present without their being received or successfully decoded during successive PRIs.
U.S. Pat. No. 3,781,881, issued Dec. 25, 1973, to J. M. Hovey, discloses a beacon target evaluator in which decoded reply signals on a video input bus are shifted serially into a plurality of N-1 series connected shift registers. Any signal appearing on the input bus that has occurred at the same relative time during any of the N-1 previous PRIs will appear at the output of the shift register related to such previous PRI. Logic means are connected to the input bus and the outputs of each of the shift registers for determining the number of times a signal on the input bus coincides with outputs from the shift registers. Means are thereby provided for establishing an M of N criterion for determining the validity of reply signals, where M is the number replies at a particular range that have appeared during N interrogations. Although the '881 evaluator does not require that the replies be received during consecutive PRIs to be deemed valid, it does require that valid replies all appear at the same range during each PRI. Consequently, replies from rapidly moving targets may not be evaluated properly and no provision is made for evaluating replies in IFF systems that contain intentional security delays.
U.S. Pat. No. 4,896,158, issued Jan. 23, 1990, to E. L. Cole, Jr. discloses a beacon fruit filter in which a Hold window and an Age window are established upon the appearance of an initial reply. The Hold window extends for five PRIs and the Age window extends for ten PRIs, suitably. The number of range coherent replies received within the Hold window are compared with a first threshold and the number of such replies received within the Age window are compared with a second threshold. Replies within the Hold window and replies within the Age window equaling or exceeding the respective first and second thresholds are output for further processing. Otherwise, the replies are discarded.
The beacon fruit filter of the '158 patent does not make reply validity determinations on the basis of all information received during N successive PRIs replies. Rather, those replies which fail to pass the first or second thresholds are discarded. A small amount of tolerance to accommodate range jitter of the responding targets is provided. However, no provision is made for replies from fast moving targets or for replies containing intentionally inserted security delays.
It is an object of the invention to provide, in a secondary radar system, a method and means for evaluating reply signals received by an interrogator wherein all replies received to N successive interrogation signals are retained for examination in parallel by a moving window spatial filter, thereby utilizing all available information relating to target range and bearing in making a reply validation decision.
It is another object of the invention to provide a method and means for evaluating reply signals received by an interrogator of a secondary radar system wherein allowances are made for security delays intentionally inserted in reply signals by responding targets, for replies from fast moving targets, and for range jitter.
Other objects and advantages of the invention will become evident as a full understanding thereof is gained from the detailed description to follow.